[iOS] Check new element before creating placeholder label (#5432)

* Check new element before creating placeholder label

* additional checks

* create placeholder once

Xamarin.Forms.Platform.iOS/Renderers/EditorRenderer.cs

@@ -31,16 +31,24 @@ namespace Xamarin.Forms.Platform.iOS
 
 		protected override void OnElementChanged(ElementChangedEventArgs<Editor> e)
 		{
+			bool initializing = false;
+			if (e.NewElement != null && _placeholderLabel == null)
+			{
+				initializing = true;
 				// create label so it can get updated during the initial setup loop
 				_placeholderLabel = new UILabel
 				{
 					BackgroundColor = UIColor.Clear
 				};
+			}
 
 			base.OnElementChanged(e);
 
+			if (e.NewElement != null && initializing)
+			{
 				CreatePlaceholderLabel();
 			}
+		}
 
 		protected internal override void UpdateFont()
 		{
@@ -63,6 +71,9 @@ namespace Xamarin.Forms.Platform.iOS
 
 		void CreatePlaceholderLabel()
 		{
+			if (Control == null)
+				return;
+
 			Control.AddSubview(_placeholderLabel);
 
 			var edgeInsets = TextView.TextContainerInset;